1. Field of the Invention
The present invention relates to an alignment changing control device for controlling electric actuators which change alignment angles of vehicle wheels.
2. Description of the Related Art
There is conventionally known four-wheel steering vehicles whose rear wheels are steered linking with the steering handle of the vehicle. For example, there is known an all-wheel independent steering device for independently controlling the operation of the alignment angles of all wheels according to the steering angle of the steering handle and the vehicle speed (see Japanese Patent Publication No. Hei 6-47388, FIG. 2 and others).
Since the all-wheel independent steering device disclosed in Japanese Patent Publication No. Hei 6-47388 employs a hydraulic actuator, a hydraulic system needs to be provided separately. In order to suppress increase in weight of the vehicle to reduce the cost, an electric actuator can be alternatively employed, instead of the hydraulic actuator.
In the steering device employing such an electric actuator, in order to protect a motor which supplies power to the electric actuator from being burn out due to the temperature rising mainly caused by Joule heat, it is necessary to measure the temperature of the motor and limit, when the temperature is equal to or higher than a predetermined temperature, the operation of the motor. To serve this purpose, a temperature sensor can be attached to the main body of the motor to measure the temperature of the motor. However, if the temperature sensor is attached to the main body of the motor, noises caused by the motor and the like will be brought into the output signal of the temperature sensor, therefore it becomes difficult to precisely measure the temperature.
To solve this problem, there is provided a controlling circuit in which the temperature of the motor is estimated using a thermal model and based on the current supplied to the motor, and the current supplied to the motor is limited based on the estimated result (see Japanese Patent Laid-Open Publication No. Hei 4-71379, page 10 line 11 to page 11 line 16 and others).
Further, in the electric actuator having such a controlling circuit, when power supplied to the motor is cut off by switching off the ignition switch, it is preferred to also cut of the power supplied to the controlling circuit in order to prevent the battery voltage from dropping. In such a case, the estimated temperature of the motor calculated by the controlling circuit is cleared however. When restarting the vehicle, the controlling circuit estimates the temperature of the motor using a predetermined initial value.
For this reason, when restarting the vehicle and driving the motor shortly after cutting off the power supplied to the motor of the electric actuator while the temperature of the motor is high, a temperature difference is generated between the actual temperature of the motor and the estimated temperature of the motor estimated by the controlling circuit using the initial value, therefore there is a concern that the motor might be burn out.
To solve this problem, there is provided a motor control device in which after the power supplied to the motor is cut off, a cutting off time (which is the time between the time when the power supplied to the motor is cut off and the time when the power of an internal battery is cut off) is calculated based on the estimated temperature of the motor, and the power of an internal battery is cut off after the calculated cutting off time has passed (see Japanese Patent Laid-Open Publication No. Hei 6-247324, claim 1, etc.).
With such a configuration, the estimated temperature can be saved until the temperature of the motor is dropped to an operation allowable temperature, and therefore even when the motor is restarted shortly after stop, the operation of the motor can be limited based on the estimated temperature, so that the motor can be prevented from burning out.
However, if the controlling circuit disclosed in Japanese Patent Laid-Open Publication No. Hei 4-71379 is applied to an electric actuator installed to a moving object such as the vehicle in a state exposed to the outside-air, since the flow of air around the electric actuator is not taken into consideration, the estimated temperature of the motor is higher than the actual temperature of the motor, and therefore the operation of the electric actuator can not be optimally limited, and the optimum electric actuator can not be selected.
Further, in the motor control device disclosed in Japanese Patent Laid-Open Publication No. Hei 6-247324, since it is necessary to provide hardware for calculating the cutting off time and to newly provide an internal battery within the motor control device, the motor control device will have a complicated structure and high cost.